This invention relates to cellular mobile telephone systems and, more particularly, to an improvement therein for indicating to the user of the mobile station the actual HOME/ROAM status thereof.
In current cellular mobile telephone systems (sometimes referred to as analog voice systems), as well as in proposed digital cellular systems, different service providers provide mobile telephone service in different geographical areas. Pursuant to FCC regulations, each service area is serviced by two providers: one a wire-line carrier, such as a conventional telephone operating company (e.g. Bell Atlantic, NYNEX, GTE, and the like) and the other a nonwire-line carrier (such as service providers who have not historically provided telephone switching service, e.g. McCaw Cellular, Rogers Cantel, Metro Mobile CTS, Inc., etc.). It is both impractical and uneconomical for a mobile telephone user to subscribe to several service providers; and subscription generally is limited to a single provider. The system to which a user subscribes is referred to as a "home" system, and the user enjoys lower costs when operating his mobile telephone within a service cell serviced by the "home" system. Even when the user moves from one cell to another in the "home" system, reduced pricing for use of "air time" still is enjoyed.
However, when a user is "handed off" from a service cell in his "home" system to a service cell that is served by a service provider to which the user does not subscribe, premium costs are incurred for the use of "air time" whether the user initiates an outgoing or receives an incoming telephone call. The service cells that are served by the provider to which the user does not subscribe are included in what is referred to as a "visited" system; and a user who moves into or through a visited system is referred to as a "roamer."
It is typical to provide a status indicator on the user's mobile telephone station to indicate the present HOME/ROAM status of the user, that is, to indicate to him if he is located in his "home" system or in a "visited" system. Since the boundaries of a cell are not clearly defined, a user located at the boundary between "home" and "visited" systems who wishes to initiate a telephone call may be willing to wait until he is well within his "home" system before doing so, thereby taking advantage of more favorable rates. The status indicator assists the user in recognizing when he has moved from a "visited" system to his "home" system and vice versa.
The determination of whether a mobile telephone user is "roaming" or is in his "home" system is based upon a system identification code SID. Upon subscribing to a service provider, the user's mobile station is encoded with an SID code which, typically, is a digital code that is stored in a nonvolatile memory (such as an EEPROM). When a mobile station is in an idle mode, power is supplied to it and it is prepared to initiate an outgoing telephone call or respond to an incoming call. In this idle mode, control messages which are transmitted periodically from the base station serving the service cell in which the mobile station is located These control messages include an SID code which represents the identification of the system that provides service to this cell. If the received SID code is equal to the stored SID code, the mobile station is located in its "home" system and a suitable indication is provided. Of course, the user of that mobile station enjoys more favorable tariffs if voice communication is established while he remains in his "home" system. Alternatively, if the received SID code differs from the stored SID code, the mobile station is a "roaming" station and a ROAM indication is provided. If the user communicates with the base station from which he has received this SID code, he is charged premium rates because of his "roaming" status.
One notable drawback of present cellular mobile telephone systems is the inability to indicate a change in HOME/ROAM status when the mobile station is in its active mode, that is, when the mobile station is in voice communication with a base station. This is because the SID code is transmitted from a base station on a channel (known as the forward control channel) that can be received by a mobile station only when that mobile station is in its idle mode. Hence, once a HOME/ROAM determination is made, it is "locked" when the mobile station changes over to its active mode. Although this failure to provide an updated indication is not particularly bothersome when the user of the active mobile station crosses from a "visited" system to a "home" system, this failure is most annoying when the user is handed off from a "home" base station to a "visited" base station because he will not be aware of the premium rates at which he is being charged. Although control messages are transmitted to and from an active mobile station, the convention of transmitting an SID code or other status indication messages during voice communication has not been recognized. Thus, a change in the HOME/ROAM status of a mobile station as it passes between "home" and "visited" systems heretofore has not been provided.
Another drawback in conventional cellular mobile telephone systems occasioned by this inability to apprise the user of an active mobile station of a change in his HOME/ROAM status relates to marketing restrictions. It would be attractive for a service provider to increase the number of subscribers to that service if arrangements can be made with the service provider in an adjacent geographic area such that subscribers to either system are treated as "home" stations when those subscribers "roam" into service cells serviced by the other provider. For example, in the metropolitan New York area, service in regions in southern New Jersey may be provided by Bell Atlantic, whereas service in regions in northern New Jersey and in New York City may be provided by NYNEX. It would be to the advantage of, for example, Bell Atlantic if it could advertise to its subscribers (and potential subscribers) that Bell Atlantic customers will be treated as "home" users in the service area serviced by NYNEX. If an appropriate commercial arrangement could be established between these service providers, the rates charged by, for example, NYNEX when a Bell Atlantic subscriber operates within a service cell serviced by NYNEX would be significantly reduced as compared to the normal "roaming" rates.
As a further marketing approach, it may be beneficial to service providers which operate in adjacent service cells, that is, in service cells which define a service boundary therebetween, to offer a "roaming" mobile station the benefit of "home" subscriber rates if the "roaming" mobile station operates in a border cell. Although service providers are supplied with identifying data representing the identities of each idle and active mobile station within each cell in its service area, such identifying data (comprised of a Mobile Station Identifying number, or MIN, and an Electronic Serial Number, or ESN) is not used for much other than verifying the authenticity of a mobile station. But, if a service provider offers different types of service, such as an arrangement with an adjacent service provider to offer reduced air time rates for particular subscribers who operate in certain service cells of the "visited" system, such MIN and/or ESN codes would serve to identify those subscribers. A suitable indication of this preferential status could be provided at the mobile station itself For example, one such subscriber who moves into particular service cells in the "visited" system may be provided with a HOME indication, thus apprising him that he may enjoy reduced tariffs even though he is "roaming" through a service area to which he is not a subscriber.
Unfortunately, the foregoing heretofore has not been possible in conventional cellular mobile telephone systems; and such selectivity of the HOME/ROAM indicator status is not presently being contemplated for digital cellular telephone systems. Thus, the marketing abilities of service providers are severely hampered by the inability to selectively control the HOME/ROAM status of a non-subscribing mobile station.